Metabolism of S-1108, a new oral cephem antibiotic, and metabolic profiles of its metabolites in humans.

The metabolism and pharmacokinetics of pivalic acid, a major metabolite of S-1108, were studied with three healthy volunteers. Concentrations of S-1006 (the active compound), pivalic acid, and pivaloylcarnitine in plasma and urine were measured after administration of S-1108. Recoveries in urine at the doses of S-1108 given (100 and 200 mg) were 33 to 41% for S-1006, 93% for total pivalic acid, and 89 to 94% for pivaloylcarnitine in 24 h, and maximum concentrations in plasma were 2 micrograms of S-1006 per ml, 1 micrograms of total pivalic acid per ml, and 2 micrograms of pivaloylcarnitine per ml after a 200-mg oral administration of S-1108. More than 90% of the pivalic acid was excreted as pivaloylcarnitine, and no measurable amount of free pivalic acid was present in urine samples, indicating that the pivalic acid liberated from S-1108 was almost quantitatively conjugated with carnitine in the human body. The level of free carnitine in plasma was unaffected by a single 200-mg administration of S-1108, whereas urinary excretion of free carnitine decreased as levels of acylcarnitine increased. The acylcarnitines were excreted primarily in the form of pivaloylcarnitine. This study clearly showed how the pivalic acid was metabolized and excreted in humans. The importance of monitoring carnitine, an essential cofactor in fatty acid metabolism, was also discussed in terms of its utilization by pivalic acid.